The present invention relates to active matrix display devices, and more particularly to drive circuitry that is located within each pixel of an active matrix display.
Arrays of organic light emitting diodes (OLEDs) are being utilized to create two-dimensional flat panel displays. As compared to conventional light emitting diodes (LEDs), which are made of compound semiconductors, the low cost and ease of patterning OLEDs makes compact, high resolution arrays practical. OLEDs can be adapted to create either monochrome or color displays and the OLEDs may be formed on transparent or semiconductor substrates.
As is known in the art, arrays of OLEDs and LEDs are typically classified as passive matrix arrays or active matrix arrays. In a passive matrix array, the current drive circuitry is external to the array, and in an active matrix array, the current drive circuitry includes one or more transistors that are formed within each pixel. An advantage of active matrix arrays over passive matrix arrays is that active matrix arrays do not require peak currents that are as high as passive matrices. High peak currents are generally undesirable because they reduce the luminous efficiency of available OLEDs. Because the transparent conducting layer of an active matrix can be a continuous sheet, active matrix arrays also mitigate voltage drop problems which are experienced in the patterned transparent conductors of passive matrices.
FIGS. 1 and 2 are depictions of active matrix pixels that are known in the prior art. It should be understood that although individual active matrix pixels are shown for description purposes, the individual active matrix pixels shown in FIGS. 1 and 2 are typically part of an array of pixels that are located closely together in order to form a display. As shown in FIGS. 1 and 2, each of the active matrix pixels includes an address line 102 and 202, a data line 104 and 204, an address transistor 106 and 206, a drive transistor 108 and 208, a storage node 110 and 210, and an OLED 112 and 212. The address lines allow the pixels to be individually addressed and the data lines provide the voltage to activate the drive transistors. The address transistors control the writing of data from the data lines to the storage nodes. The storage nodes are represented by capacitors, although they need not correspond to separate components because the gate capacitance of the drive transistors and the junction capacitance of the address transistors may provide sufficient capacitance for the storage nodes. As shown, the OLEDs are connected to a drive voltage (VLED) and the current that flows through the OLEDs is controlled by the drive transistors. When current is allowed to flow through the drive transistors, the OLEDs give off light referred to as a luminous flux, as indicated by the arrows 114 and 214.
Referring to FIG. 1, PMOS transistors are preferred when the cathode of the OLED 112 is grounded, and referring to FIG. 2, NMOS transistors are preferred when the anode of the OLED 212 is connected to the supply voltage (VLED). Utilizing the PMOS and NMOS transistors as shown in FIGS. 1 and 2 makes the gate to source voltages of the drive transistors 108 and 208 insensitive to voltage drops across the OLEDs, thereby improving the uniformity of the light 114 and 214 that is given off by the OLEDs.
The operation of the prior art active matrix pixels is described with reference to the active matrix pixel configuration shown in FIG. 2, although the same concepts apply to the active matrix pixel of FIG. 1. The active matrix pixel shown in FIG. 2 serves as an analog dynamic memory cell. When the address line 202 is high, the data line 204 sets the voltage on the storage node 210, which includes the gate of the drive transistor 208. When the voltage on the storage node exceeds the threshold voltage of the drive transistor, the drive transistor conducts causing the OLED 212 to emit light 214 until the voltage on the storage node drops below the threshold voltage of the drive transistor, or until the voltage on the storage node is reset through the address transistor 206. The voltage on the storage node will typically drop due to leakage through the junction of the address transistor and through the gate dielectric of the drive transistor. However, with sufficiently low leakage at the address and drive transistors and high capacitance at the storage node, the current through the OLED is held relatively constant until the next voltage is set on the storage node. For example, the voltage is typically reset at a constant refresh interval as is known in the art. The storage node is represented as a capacitor in order to indicate that sufficient charge must be stored on the storage node to account for leakage between refresh intervals. As stated above, the capacitor does not necessarily represent a separate component because the gate capacitance of the drive transistor and the junction capacitance of the address transistor may suffice.
In the active matrix pixel of FIG. 2, the voltage on the storage node 210 determines the intensity of the light 214 that is generated by the OLED 212. If the intensity-current relationship of the OLED and gate voltage-current relationship of the drive transistor 208 are known, according to one method, the desired intensity of light is generated by placing the corresponding voltage on the storage node. Setting the voltage on the storage node is typically accomplished by utilizing a digital to analog converter to establish the voltage on the corresponding data line 204. In an alternative method, the storage node is first discharged by grounding the data line, and then the data line is set to the CMOS supply voltage (Vdd). Utilizing the latter method, the address transistor 202 functions as a current source, charging the storage node until the storage node is isolated by setting the address line low. The latter method offers the benefit of not requiring a digital to analog converter on each data line. However, one disadvantage of the latter method is that the storage node capacitance within a single pixel is a non-linear function of the voltage when supplied by the gates and junctions of the transistors. Another disadvantage is that the storage node capacitance of each pixel varies among the pixels in an array.
As described above, in order to obtain the desired luminous flux from the OLED 212 of FIG. 2, the voltage on the data line 204 is adjusted to control the current through the drive transistor 208. Unfortunately, current flow through the drive transistor also depends on characteristics of the drive transistor, such as its threshold voltage and transconductance. Large arrays of drive transistors, as required to make a high-resolution display, exhibit variations in threshold voltage and transconductance that often cause the drive currents of the OLEDs to differ for identical control voltages, which in turn causes a display to appear non-uniform. In addition, different OLEDs emit different intensities of light even when driven with identical currents. Furthermore, the light intensity for a specified drive current drops as an OLED ages and different OLEDs can degrade at different rates, again causing a display to appear non-uniform.
Active matrix pixels are preferably implemented with a silicon substrate instead of a transparent dielectric substrate because transparent dielectric substrates require the transistors to be built as thin film devices. It is difficult to obtain a tight distribution of threshold voltages in large arrays of thin-film transistors especially as more transistors are needed to make the luminous flux from each pixel insensitive to threshold variations. With a silicon substrate, addressing, driving, and other circuit functions, can be easily integrated, particularly if the substrate and process are compatible with CMOS technology. Although known active matrix pixel technology is compatible with older CMOS technology, OLEDs require higher voltages than dense CMOS can tolerate, while dense CMOS is desirable for the small pixels that are required for high-resolution color displays.
A technique that has been utilized to produce a uniform luminous flux in other LED applications involves providing feedback to an LED through the use of a photosensor. Providing feedback to an LED utilizing known techniques typically involves amplifiers and comparators, which require much more circuitry than can fit into a single pixel of, for example, a high-resolution color display.
As described above the intensity of light generated by an OLED is influenced by the voltage supplied to the storage node and by characteristics of the drive transistors and OLEDs, which can vary from pixel to pixel. The differences in the characteristics of the pixels can produce non-uniform light intensities. In addition, as OLEDs age, the degree of non-uniformity may change. As a result, what is needed is a system and method for individually driving each pixel in an active matrix array that provides uniform luminous flux while meeting the size limitations of active matrix displays.
An active matrix pixel within an active matrix display includes a photodiode that is optically connected to a light emitting diode within the pixel in order to detect a portion of the luminous flux that is generated by the light emitting diode. The photodiode discharges excess charge within the pixel in response to the detected portion of luminous flux. Once the excess charge is discharged, the light emitting diode stops emitting light. In an embodiment, the gate of a drive transistor is controlled by the charge on a storage node. If the charge on the storage node sets a voltage that exceeds the threshold voltage of the drive transistor then the drive transistor conducts. The amount of charge on the storage node above that which is needed to set the threshold voltage is referred to as the excess charge.
As long as the excess charge is present, the drive transistor conducts and the light emitting diode emits a luminous flux. However, when the excess charge is discharged from the storage node the voltage on the storage node drops below the threshold voltage of the drive transistor, the drive transistor stop conducting, and the light emitting diode stops emitting a luminous flux. The amount of luminous flux generated by the light emitting diode can be controlled by controlling the amount of excess charge that is placed on the storage node. Because the excess charge on the storage node is discharged in proportion to the amount of luminous flux that has been received by the photodiode, the luminous flux of the pixel is insensitive to the variation in characteristics of the drive transistor and the light emitting diode. The insensitively to the variation within each pixel of an active matrix pixel array allows the array to provide a more uniform luminous flux across the display.
In an embodiment, the active matrix pixel includes an address line, a data line, an address transistor a drive transistor, a storage node, an OLED, and a photodiode. The address line allows the pixel to be individually addressed and the data line provides the voltage to activate the drive transistor. The capacitor does not necessarily represent a separate physical component, but can represent the capacitance of the gates and junctions connected to the storage node. The drive transistor conducts as long as the voltage on the storage node exceeds the corresponding threshold voltage of the drive transistor. It should be understood that although a single active matrix pixel is described, the single pixel is typically part of an array of pixels that are located closely together in order to form a display.
The photodiode is optically coupled to the OLED so that the photodiode can detect a portion of the light that is generated by the OLED. The photodiode discharges the excess charge that is present on the storage node at a rate that is proportional to the luminous flux that is generated by the OLED. Because the photodiode discharges the excess charge on the storage node in proportion to the luminous flux of the OLED, the drive transistor and the OLED are turned off when the integrated flux detected by the photodiode has reached a value that is equivalent to the excess charge that is on the data line.
In operation, the address line of the active matrix pixel is set high for a period of time that charges the storage node with a desired amount of excess charge. Once the storage node is sufficiently charged, the address line is set low, effectively isolating the storage node from the data line. The drive transistor begins to conduct current as soon as the threshold voltage of the drive transistor is exceeded. Current conducting through the drive transistor causes the OLED to give off a luminous flux. A portion of the luminous flux is detected by the photodiode and in response, the photodiode discharges the charge on the storage node at a rate that id directly proportional to the luminous flux that is detected by the photodiode. At the point where the integrated value of the detected luminous flux equals the excess charge on the storage node, the voltage on the storage node drops below the threshold voltage of the storage node. Once the voltage on the storage node drops below the threshold voltage of the drive transistor, current stops flowing through the drive transistor and the OLED stops generating light.
In an embodiment, an additional transistor, referred to as the isolation transistor, is connected to the logical complement of the address line. Connecting the isolation transistor to the logical complement of the address line, prevents the isolation transistor from turning on the drive transistor when the storage node is being written from the data line. With the isolation transistor in place, the action of the photodiode controls the flow of current through the drive transistor and the OLED, and the OLED does not emit light until the address line goes low.
In an embodiment, the active matrix pixel may utilize a bipolar transistor as the drive transistor. The role of the bipolar transistor is solely to withstand VLED and the bipolar transistor does not need to provide high gain or operate at high frequencies.